Techniques (pattern-forming techniques) in which a fine pattern is formed on top of a substrate, and a lower layer beneath that pattern is then fabricated by conducting etching with this pattern as a mask are widely used in the production of semiconductor devices and liquid crystal display devices. These types of fine patterns are usually formed from an organic material, and are formed, for example, using a lithography method or a nanoimprint method or the like. In a lithography method, for example, a resist film is formed on a support such as a substrate using a resist material containing a base component such as a resin, and the resist film is then subjected to selective exposure of radial rays such as light or electron beam, followed by development, thereby forming a resist pattern having a predetermined shape on the resist film. Using this resist pattern as a mask, a semiconductor device or the like is produced by conducting a step in which the substrate is processed by etching.
The aforementioned resist materials can be classified into a positive type and a negative type. A resist material in which the exposed portions exhibit increased solubility in a developing solution is called a positive type, and a resist material in which the exposed portions exhibit decreased solubility in a developing solution is called a negative type.
In general, an aqueous alkali solution (alkali developing solution) such as an aqueous solution of tetramethylammonium hydroxide (TMAH) is used as the developing solution. Alternatively, organic solvents such as aromatic solvents, aliphatic hydrocarbon solvents, ether solvents, ketone solvents, ester solvents, amide solvents and alcohol solvents are used as the developing solution (for example, see Patent Documents 1 and 2).
In recent years, advances in lithography techniques have led to rapid progress in the field of pattern miniaturization.
Typically, these miniaturization techniques involve shortening the wavelength (increasing the energy) of the exposure light source. Conventionally, ultraviolet radiation typified by g-line and i-line radiation has been used, but nowadays KrF excimer lasers and ArF excimer lasers are now starting to be introduced in mass production. Furthermore, research is also being conducted into lithography techniques that use an exposure light source having a wavelength shorter (energy higher) than these excimer lasers, such as electron beam (EB), extreme ultraviolet radiation (EUV), and X ray.
As shortening of the wavelength of the exposure light source progresses, it is required to improve various lithography properties of the resist material, such as the sensitivity to the exposure light source and the resolution capable of reproducing patterns of minute dimensions. As resist materials which satisfy such requirements, chemically amplified resists are known.
As a chemically amplified resist, a composition is generally used, which includes a base component that exhibits a changed solubility in a developing solution under the action of acid and an acid generator component that generates acid upon exposure. For example, when the above developing solution is an alkali developing solution (when the process is an alkali developing process), as the base component, a base component that exhibits increased solubility in an alkali developing solution by the action of acid is used.
Conventionally, a resin (base resin) is mainly used as the base component of a chemically amplified resist composition. Currently, resins that contain structural units derived from (meth)acrylate esters within the main chain (acrylic resins) are the mainstream as base resins for chemically amplified resist compositions that use ArF excimer laser lithography, as they exhibit excellent transparency in the vicinity of 193 nm.
Here, the term “(meth)acrylic acid” is a generic term that includes either or both of acrylic acid having a hydrogen atom bonded to the α-position and methacrylic acid having a methyl group bonded to the α-position. The term “(meth)acrylate ester” is a generic term that includes either or both of the acrylate ester having a hydrogen atom bonded to the α-position and the methacrylate ester having a methyl group bonded to the α-position. The term “(meth)acrylate” is a generic term that includes either or both of the acrylate having a hydrogen atom bonded to the α-position and the methacrylate having a methyl group bonded to the α-position.
In general, the base resin contains a plurality of structural units for improving lithography properties and the like. For example, a structural unit having a lactone structure and a structural unit having a polar group such as a hydroxy group are used, as well as a structural unit having an acid decomposable group which is decomposed by the action of an acid generated from the acid generator to form an alkali soluble group (for example, see Patent Document 3). When the base resin is an acrylic resin, as the acid decomposable group, in general, resins in which the carboxy group of (meth)acrylic acid or the like is protected with an acid dissociable group such as a tertiary alkyl group or an acetal group are used.
As a technique for further improving the resolution, a lithography method called liquid immersion lithography (hereafter, frequently referred to as “immersion exposure”) is known in which exposure (immersion exposure) is conducted in a state where the region between the lens and the resist layer formed on a wafer is filled with a solvent (a immersion medium) that has a larger refractive index than the refractive index of air (see for example, Non-Patent Document 1).
According to this type of immersion exposure, it is considered that higher resolutions equivalent to those obtained using a shorter wavelength light source or a larger NA lens can be obtained using the same exposure light source wavelength, with no lowering of the depth of focus. Furthermore, immersion exposure can be conducted by applying a conventional exposure apparatus. As a result, it is expected that immersion exposure will enable the formation of resist patterns of higher resolution and superior depth of focus at lower costs. Accordingly, in the production of semiconductor devices, which requires enormous capital investment, immersion exposure is attracting considerable attention as a method that offers significant potential to the semiconductor industry, both in terms of cost and in terms of lithography properties such as resolution.
Immersion exposure is effective in forming patterns having various shapes. Further, immersion exposure is expected to be capable of being used in combination with currently studied super-resolution techniques, such as phase shift method and modified illumination method. Currently, as the immersion exposure technique, technique using an ArF excimer laser as an exposure source is being actively studied. Further, water is mainly used as the immersion medium.
Because it is necessary to impart water repellency to the obtained resist film in the immersion exposure, resist compositions for immersion exposure which contain a fluorine-containing compound have been reported (see, for example, Non-Patent Document 1).
Active research and development of fluorine-containing compounds have been conducted in various fields including the resist materials for immersion exposure described above for their properties such as water repellency and transparency. For example, in the field of resist materials, fluorine-containing polymers that include a structural unit containing a fluorine atom have been used in recent years (see Patent Document 4). Examples of the Patent Document 4 show that the specific fluorine-containing compound contributes to the defect reduction in the formation of the pattern using the immersion exposure, but is not limited therein. It is known that fluorine-containing compounds contribute to improvement of lithography properties such as the defect reduction (see Patent Documents 5 and 6).
As a lithography technique which has been recently proposed, a double patterning method is known in which patterning is conducted two or more times to form a resist pattern (for example, see Non-Patent Documents 2 and 3). There are several different types of double patterning process, for example, (1) a method in which a lithography step (from application of resist compositions to exposure and developing) and an etching step are performed twice or more to form a pattern and (2) a method in which the lithography step is successively performed twice or more. According to the double patterning method, a resist pattern with a higher level of resolution can be formed, as compared to the case where a resist pattern is formed by a single lithography step (namely, a single patterning process), even when a light source with the same exposure wavelength is used, or even when the same resist composition is used. Furthermore, double patterning process can be conducted using a conventional exposure apparatus.
Moreover, a double exposure process has also been proposed in which a resist film is formed, and the resist film is subjected to exposure twice or more, followed by development to form a resist pattern (for example, see Patent Document 7). Like the double patterning process described above, this type of double exposure process is also capable of forming a resist pattern with a high level of resolution, and also has an advantage in that fewer number of steps is required than the above-mentioned double patterning process.
In a positive tone development process using a positive type, chemically amplified resist composition (i.e., a chemically amplified resist composition which exhibits increased alkali solubility in an alkali developing solution upon exposure) in combination with an alkali developing solution, as described above, the exposed portions of the resist film are dissolved and removed by an alkali developing solution to thereby form a resist pattern. The positive tone process using a combination of a positive chemically amplified resist composition and an alkali developing solution is advantageous over a negative tone development process in which a negative type, chemically amplified resist composition is used in combination with an alkali developing solution in that the structure of the photomask can be simplified, a satisfactory contrast for forming an image can be reliably obtained, and the characteristics of the formed resist pattern are excellent. For these reasons, currently, positive-tone development process using a combination of a positive chemically amplified resist composition and an alkali developing solution is mainly employed in the formation of an extremely fine resist pattern.